Level shifters are often required for a Liquid Crystal Display (LCD) panel to control the voltage and charge on control lines for the LCD panel. Drivers may reverse the voltage on the control lines, which include a distributed capacitive load. In general, each control line is driven from a high power supply to a low power supply, through a switch. Since these control lines are operated at high frequency the resulting power dissipation is inconveniently large, which may result in self-heating and high power consumption.
However, since the control lines are switched in pairs, each switching to the voltage of the other control line in the pair, there is a possibility to save some of the energy by connecting the two distributed capacitances together. High voltage level shifters for LCD panels sometimes employ a technique known as “charge sharing” to reduce the amount of power required to exchange the voltages applied to the large distributed capacitances. When the capacitive load is charged, for example to a positive state, a voltage equal to the sum of a high side and a low side voltage is applied to the load capacitance.
When it is desired to reverse the polarity of the applied voltage, switches operate to connect the positive voltage to the low side and the negative voltage to the high side. This makes the load voltage reverse and requires, from a power supply, all the energy to charge the capacitance to twice the sum of the high and low voltages.
If, during a short interval, the capacitive load is disconnected from the power supplies the two ends of the capacitor may be connected together and the charge and energy made approximately zero. As a result, this “charge sharing” operation may reduce by half the energy required from the supplies when the voltages on the capacitors are reversed because the charge sharing brings the voltages part of the way toward their new voltage in the opposite direction.
FIG. 1 is a schematic diagram of a conventional charge sharing circuit. A high supply voltage (Vhigh) is coupled to a first position of a switch S1 and a third position of a switch S2. A low supply voltage (Vlow) is coupled to a third position of switch S1 and a first position of switch S2. Second positions of switches S1 and S2 are positions wherein the switches are open and not connected to either the high supply voltage or the low supply voltage. Resistor R1 represents parasitic resistance that may exist in the system. Switch S1 connects to a first capacitor C1 representing the distributed load on one control line of the LCD panel and switch S2 connects to a second capacitor C2 representing the distributed load on another control line of the LCD panel. In many systems, the other sides of capacitors C1 and C2 are connected together to a common signal (Vcommon) that may be driven to a midpoint voltage, a ground, or other suitable voltage. Switch S3 is coupled in series with resistor R1 and between the first capacitor C1 and the second capacitor C2.
In operation, if the switches S1 and S2 are in position 1, the two series capacitors, having a net value of C/2, will be charged to Vhigh−Vlow and the stored energy can be represented as:(1/2)(C/2)(Vhigh−Vlow)2 
If the switches S1 and S2 are toggled to position 3, the voltage across the capacitors will be reversed. One way to think about the result is to note that all the energy stored in capacitors C1 and C2 must be removed just to get the voltages to zero. Then the energy in capacitors C1 and C2 must be replaced with a like amount to build up the same voltage in the reverse direction. This total change in energy can be represented as:(C/2)(−Vhigh+Vlow)2 
This energy is provided by the voltage supplies Vhigh and Vlow and the power required over time can be represented as approximately:(Reversals per second)( C/2)(−Vhigh+Vlow)2.
However, if there is a time period between when the switches S1 and S2 are in position 1 and position 3, the switches S1 and S2 can be placed in position 2 where they are both open. Switch S3 can then be closed to connect the first capacitor C1 to the second capacitor C2 and they will “share” their charge such that they reach about a mid-point (minus any energy dissipated in resistor R1) between \Thigh and Vlow.
The stored energy will be switched between the capacitors C1 and C2 and will not need to come from the power supplies. Switch S3 can then be opened and the switches S1 and S2 can be placed in either position 1 or position 2 to charge capacitors C1 and C2 the rest of the way to their respective voltage levels. As a result, the system ends up with a net loss of only about half the energy required if the capacitor ends are not shorted through the resistor R1. The result of this “charge sharing” through the resistor R1 is that the energy from the supplies per reversal is cut about in half, thereby cutting the operating power by about a half.
However, energy consumption can still be high and the inventors have appreciated that there is a need to further reduce the energy required to drive LCD panels.